Typically, satellite antennas operate in either linear or circular polarizations. Therefore, antennas are designed to have either linear or circular polarization. In some instances during orbit it is desirable to switch the polarization of a satellite's antenna from linear to circular or vice versa.
For antennas operating with linear polarizations, the orientation of the polarization is fixed before the satellite is launched. The fixed linear polarization is a problem in situations where it becomes necessary to modify the orientation of the linear polarization while the satellite is in orbit. For example, when a satellite is moved from one orbit slot to another, its orientation to ground is changed. Another example, is when a user of a particular satellite is changed.
In the prior art complex methods are known that allow arbitrary polarization. One method is to separate a signal into two orthogonal polarizations. The two components are used directly for linear polarization. However, the antenna feed must be properly oriented to the desired polarization.
Reorientation of the linear polarization is accomplished by using two 90.degree. polarizers back-to-back. A polarizer is located near an ortho-mode transducer that converts circular polarization to linear polarization, or linear to circular depending on whether it is used in receive mode or transmit mode. A second polarizer is located near the antenna feed and is oriented to provide the proper linear polarization orientation upon output of the signal, or to generate circular polarization upon receiving a particular linear polarization.
When converting linear polarization to circular polarization, the linear signal must be decomposed into two orthogonal components that are then recombined with a 90 degree phase shift in one of the components. To select whether linear or circular polarization is to be used, a separate path is chosen to process the signal and achieve the desired polarization.
An alternative approach includes two feeds for one antenna. One feed is for linear polarization and the other feed is for circular polarization. The circular polarization feed must be integrated with a polarizer. The appropriate feed is chosen depending on the desired polarization.
A problem with both of the methods described above is that a switching method is required. The need for separate feeds requires switching between feeds in order to select the polarization. Likewise it is necessary to have switchable paths with the decomposition of the signal into two orthogonal components.